the way and reason it works .. is actually quite simple.... especially on cars that rely heavily on electronics as much as ours do.
basically what these kits do is take all the engine and chassis grounds and route them directly too the battery instead of relying on the chassis to transmit the ground there.
any rust or impurities could put added resistance in the ground side of chassis/engine grounds there by limiting or "slowing" down the electricity flow throughout the car. it'* a principle used heavily in Car audio and Marine ... does Chassis ground work... yes ofcourse your cars working.... directy to battery ground is better, ... why don't OEM do it right off the bat.... 12 or more feet of wire x how many 100'* of thousands of cars a year.... yeah.
typically in electronics you will find OEM does only what they need to work to spec.
as for the positive side of the "wiring" kits.... I do beleive they use a capacity of some form ... just to keep equal voltage at all times, stop nasty momentary spikes... etc... again.. does OEM work... ofcourse.. your car'* running... will this work better... yep... why don't OEM do it... cause their design works to spec.